Usually the photoacoustic signal level is low; therefore at signal detection it is very important to reject external noise effectively. The PA signal is generated in a chamber, which has openings for free gas-flow, however in ideal case it ought to be closed for achieving the highest possible level acoustical signal generation. In order to emulate closed gates, the measuring chamber requires high acoustical impedance at its openings. Well designed acoustical networks, consisting of tubes and cavities may ensure appropriate noise filtering and acoustically hard chamber termination at the same time. On the basis of electrical transmission line analogy cost-effective and easy to realize acoustical filter networks can be designed. These acoustical filters - in combination with low noise microphone amplifier and electronic filters in the signal processing chain - result in clear PA signal for gas analysis. In the paper the theory of acoustical transmission line filter design for PA applications and a software implementation will be presented.